Wednesday, August 8, 2012

Anonymous said: Hi! I stumbled upon your tumblr, and I'd like to start off by saying how amazing it is, and thank you for making this tumblr! Moreon to my issue, i'm currently studying crude oil in Chemistry. Could you please help me understand"cracking" in terms of crude oil? From what I understand, 'cracking' is the CHEMICAL process of breaking down large molecules into smaller ones. And they 'crack' crude oil to refine it into petroleum; fractional distillation being a PHYSICAL process. More info please?

It sounds like you’re a bit confused between fractional distillation and cracking. It’s true that cracking is a chemical process and fractional distillation is a physical process, but by saying that I mean to show you that they’re two entirely different processes.

When crude oil is first extracted from the ground, is made up of a variety of different hydrocarbons (chemical compounds that only consist of carbon and hydrogen), some very short (ethene) and some long (decane), and is entirely useless in this state.  Hydrocarbons can be separated into two groups: alkanes and alkenes. An alkane is saturated, meaning it holds as many hydrogen atoms as possible, whereas an alkene is unsaturated and contains a double carbon bond. 

Fractional distillation serves to separate the longer hydrocarbons from the shorter hydrocarbons by their boiling points. This works because the longer the hydrocarbon, the higher the boiling point and viscosity and the lower the flammability.

Fractional distillation takes place as follows:

  1. Crude oil is vapourised and fed into the bottom of the fractionating column.
  2. As the vapour rises up the column, the temperature falls.
  3. Fractions with different boiling points condense at different levels of the column and can be collected.
  4. The fractions with high boiling points (long chain hydrocarbons) condense and are collected at the bottom of the column
  5. Fractions with low boiling points (short chain hydrocarbons) rise to the top of the column where they condense and are collected.

To see a diagram of the fractional distillation process, click here.

Cracking on the other hand, breaks long alkanes down into shorter, more useful alkane and alkene molecules. It requires a catalyst (a substance that causes or accelerates a chemical reaction without itself being affected) and a high temperature. This is done mainly to assuage the high industrial demand for the shorter molecules. The alkenes are typically converted into polymers (plastics) while the alkanes are sought after as a fuel source. Cracking is an example of a thermal decomposition reaction.

I hope that helps clear up some of your confusion.